WASHINGTON — Less than a quarter of ObamaCare enrollments are in the closely watched 18-to-24 age group — well short of expectations, which could pressure insurance companies to jack up rates down the road.

In its first release of demographic data about the new national health plan, the administration on Monday said just 24 percent of those who signed up are young people. The unofficial target is 39 percent.

“The ObamaCare exchange is skewing older,” said Paul Howard of the Manhattan Institute’s Center for Medical Progress. “Older patients tend to be sicker with more chronic conditions. It’s a worse risk pool than what was expected.”

Administration officials insisted they were pleased with the numbers and said they expect a late rush by the young and healthy to get coverage before the March 31 deadline.

“We’re only halfway through the open-enrollment period and we expect an increase in the proportion of young adults as we go forward,” said Mike Harsh of Health and Human Services.